Yes, there is a condition called "Body Dysmorphic Disorder" (BDD) that can be related to what you described. Body Dysmorphic Disorder is a mental health disorder characterized by a preoccupation with perceived defects or flaws in one's appearance. These perceived defects are often not noticeable to others or are slight, but individuals with BDD believe they are highly noticeable and are extremely distressing to them.
In the case you mentioned, an individual with BDD may believe themselves to be too thin, even when they are unhealthily overweight. This distortion in body image can lead to obsessive thoughts, anxiety, and behaviors aimed at trying to change or hide the perceived flaw.
It's essential to note that Body Dysmorphic Disorder is different from eating disorders like anorexia nervosa, which primarily involve distorted body image related to body weight and shape. While individuals with anorexia nervosa might genuinely be underweight and still perceive themselves as overweight, those with BDD focus on specific perceived flaws in their appearance, regardless of their actual weight or body shape.
Like eating disorders, Body Dysmorphic Disorder requires professional evaluation and treatment by mental health experts, such as psychologists or psychiatrists. Cognitive-behavioral therapy and medication can be part of the treatment plan to help individuals with BDD manage their symptoms and improve their quality of life. If you or someone you know is experiencing symptoms of BDD, it's crucial to seek help from a qualified healthcare professional.